Correcting 3D effects in phase shifting masks using sub-resolution features

ABSTRACT

Using phase shifting on a mask can advantageously improve printed feature resolution on a wafer, thereby allowing greater feature density on an integrated circuit. Phase shifting can create an intensity imbalance between 0 degree and 180 degree phase shifters on the mask. An improved method of designing an alternating PSM to minimize this intensity imbalance is provided. Sub-resolution features, called “blockers”, can be incorporated in the alternating PSM design. Specifically, blockers can be formed in the 0 degree phase shifters. In this configuration, the intensity associated with the 0 degree phase shifters approximates the intensity associated with the corresponding 180 degree phase shifters. Intensity balancing using blockers retains image contrast, thereby ensuring printed feature quality.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of alternating phase-shifting masks,and in particular to a method of correcting three-dimensional (3D)effects in alternating phase-shifting masks using sub-resolutionfeatures.

2. Description of Related Art

To fabricate an integrated circuit (IC), a physical representation ofthe features of the IC, e.g. a layout, is transferred onto a pluralityof masks. Note that as used herein, the term “mask” includes “reticles”.The features make up the individual components of the circuit, such asgate electrodes, field oxidation regions, diffusion regions, metalinterconnections, and so on. A mask is generally created for each layerof the IC. To create a mask, the data representing the layout for acorresponding IC layer can be input into a device, such as an electronbeam machine, which writes IC features onto the mask. Once a mask hasbeen created, the pattern on the mask can be transferred onto the wafersurface using a lithographic process.

Lithography is a process whose input is a mask and whose output includesthe printed patterns on a wafer. As printed patterns on the IC becomemore complex, a need arises to decrease the feature size. However, asfeature sizes shrink, the resolution limits of current optical-basedlithographic systems are approached. Specifically, one type oflithographic mask includes clear regions and opaque regions, wherein thepattern of these two regions defines the features of a particularsemiconductor layer. Under exposure conditions, diffraction effects atthe transition of the transparent regions to the opaque regions canrender these edges indistinct, thereby adversely affecting theresolution of the lithographic process.

Various techniques have been proposed to improve this resolution. Onesuch technique, phase-shifting, uses phase destructive interference ofthe waves of incident light. Specifically, phase-shifting shifts thephase of a first region of incident light waves approximately 180degrees relative to a second, adjacent region of incident light waves.In this manner, the projected images from these two regionsdestructively interfere where their edges overlap, thereby improvingfeature delineation and allowing greater feature density on the IC. Amask that uses such techniques is called a phase shifting mask (PSM).

In one type of PSM, called an alternating PSM, apertures between closelyspaced features are processed so that light passing through any apertureis 180 degrees out of phase from the light passing through an adjacentaperture. FIGS. 1A and 1B illustrate one embodiment of an alternatingPSM 100 including closely spaced opaque (e.g. chrome or some otherabsorbing material) features 101, 102, 103, and 104 formed on atransparent, e.g. quartz, substrate 105. Thus, apertures 106, 107, and108 are formed between features 101-104.

To provide the phase shifting in this embodiment, the areas of substrate105 under alternating apertures can be etched, thereby causing thedesired 180 degree phase shift. For example, substrate 105 can be etchedin the area defined by aperture 107 to a predetermined depth. In thismanner, the phase shift of light passing through aperture 107 relativeto light passing through apertures 106 and 108 is approximately 180degrees.

Unfortunately, the use of an alternating PSM can introduce an intensityimbalance problem. FIG. 1C illustrates a graph 130 that plots intensity(0 to 1.0) versus position on alternating PSM 100. In graph 130,waveforms 131 that are shown nearing 1.0 intensity correspond toapertures 106 and 108, whereas waveform 132 that is shown atapproximately 0.84 intensity corresponds to aperture 107. The intensityimbalance between the 180 degree phase shifting region (i.e. aperture107) and the 0 degree phase shifting regions (i.e. apertures 106 and108) is caused by the trench cut into substrate 105, thereby causingdiffraction in the corners of aperture 107 and degrading the intensityof the corresponding waveform. This industry-recognized diffractioneffect is called a three-dimensional (3D) effect.

Intensity imbalance can adversely affect printing features and overlayon the wafer. Typically, a feature on a binary mask has a pair ofcorresponding phase shifting regions on an alternating PSM. For example,referring to FIG. 1D, a feature 140 can have a corresponding 0 degreephase shifting region (also called a phase shifter herein) 141 placedrelative to one side of feature 140 and a corresponding 180 degree phaseshifter 142 placed relative to the other side of feature 140. Ofinterest, if phase shifters 141 and 142 are the same size, the electricfield associated with phase shifter 141 is stronger than the electricfield associated with phase shifter 142, thereby resulting in themaximum interference of these fields to occur to the right of centerline143 on feature 140. Thus, under these conditions, feature 140 willactually print on the wafer to the right of the desired location asshown by dotted lines indicating the printed location of feature 150 andits associated centerline 153.

Moreover, any defocus in the system can exacerbate the 3D effect andcause significant deviation from desired feature placement on the wafer.Because any wafer production line requires at least some acceptablerange of defocus, e.g. typically within 0.4 microns, feature placementis frequently adversely affected when using an alternating PSM.Therefore, those in the industry have proposed various methods toaddress the intensity imbalance problem.

In one proposed method shown in FIG. 1E, an additional etching step canbe performed on substrate 105, thereby providing an undercut etch 160 offeatures 101-104. Undercut etch 160 increases the intensity byattempting to localize the diffraction effects under features 101-104.Unfortunately, under-cut etch 160 can also create mechanical instabilityof features 101-104 on the mask. Specifically, the greater the under-cutetch, the greater the probability of mechanical (e.g. chrome)instability during subsequent processing steps, such as mask cleaning.Thus, undercut etch 160 provides an incomplete solution with thepotential of causing complete mask failure.

Another potential solution (not shown) includes biasing the size of the180 degree phase-shifting region to be larger than the 0 degreephase-shifting region, as described in U.S. Pat. No. 6,670,082, whichissued on Dec. 30, 2003 to the assignee of the present application,Numerical Technologies, Inc. This method ensures mechanical stability,but does typically require determining the appropriate bias for aplurality of 180 degree phase-shifting regions.

Another potential solution (not shown) includes providing a dual trenchstructure in the PSM, i.e. both the 0 degree and 180 degree phaseshifters are formed using trenches. Using this structure, both phaseshifters suffer from diffraction effects, which can minimize or eveneliminate the undesirable intensity imbalance. Unfortunately, formingthe additional trenches in the substrate adds significantly more time tothe manufacturing operation, e.g. typically doubling the time, therebyundesirably increasing manufacturing cost.

Yet another potential solution (not shown) includes sloping thesidewalls of the 180 degree phase shifters and providing a layer ofchrome on such sloped sidewalls, thereby minimizing diffraction effects.However, this type of mask, called sidewall chrome alternating aperturemask (SCAAM), also suffers from significantly higher manufacturing costthan the standard PSM. Moreover, as features get smaller, theappropriate angle for the sloping sidewalls can become a limitingfactor. That is, the angle may result in a de facto minimum featuresize.

Therefore, a need arises for a method of correcting 3D effects ofphase-shifting masks while ensuring mechanical stability andmanufacturing efficiency.

SUMMARY OF THE INVENTION

Using phase shifting on a mask can advantageously improve printedfeature resolution on a wafer, thereby allowing greater feature densityon an integrated circuit. An alternating phase shifting mask (PSM) haspairs of apertures, wherein light passing through one aperture is 180degrees out of phase from light passing through the other aperture. Forease of reference, these apertures are called 0 degree phase shiftersand 180 degree phase shifters herein. To provide the appropriate phaseshift, the mask substrate is etched in the area associated with the 180degree phase shifters. The resulting trenches in the substrate canresult in an intensity imbalance between the 180 degree phase shiftersand the 0 degree phase shifters.

This intensity imbalance, without correction, can undesirably change thelocations of printed features on the wafer. Known solutions to thisintensity imbalance problem can cause mechanical instability on the maskor decrease feature density on the integrated circuit. Therefore, animproved method of designing an alternating PSM to minimize thisintensity imbalance is provided.

In accordance with one aspect of the invention, sub-resolution features,called “blockers”, can be incorporated in the alternating PSM design.Specifically, blockers can be formed in the 0 degree phase shifters. Inthis configuration, the intensity associated with the 0 degree phaseshifters approximates the intensity associated with the corresponding180 degree phase shifters. Of importance, this intensity balancingretains image contrast, thereby ensuring printed image quality.

In one embodiment, the length of the blocker can be “grown” until thedesired amount of intensity imbalance correction is achieved. Notably,the size of each blocker can be determined using asoftware-implementation. Thus, blocker size can be easily customized foreach pair of phase shifters. This process flexibility can be used tocreate a uniform intensity imbalance error on the mask.

At this point, a mask-based solution can be used to correct for thatuniform intensity imbalance error. Exemplary mask-based solutionsinclude undercutting or biasing of the 180 degree phase shifters.Fortunately, the use of the blockers can significantly reduce the amountof undercutting necessary (if needed at all), thereby advantageouslyreducing the minimum chrome specification as well as the risk of chromedefects that could occur during the mask cleaning process.

In another embodiment, the number of the blockers within the 0 degreephase shifter can be increased until the desired amount of intensityimbalance correction is achieved. In yet another embodiment, if a 180degree phase shifter includes a sub-resolution feature, then the blockerin the corresponding 0 degree phase shifter can be sized to be largerthan the sub-resolution feature.

Thus, an alternating PSM can include a 180 degree phase shifter, a 0degree phase shifter corresponding to the 180 degree phase shifter, andat least one sub-resolution feature formed in the 0 degree phase shifterto minimize an intensity imbalance with the 180 degree phase shifter.The alternating PSM can further include an undercut or a bias in the 180degree phase shifter. In an embodiment where a sub-resolution feature isformed in the 180 degree phase shifter, the blocker in the 0 degreephase shifter can be formed to be larger than the sub-resolution featureformed in the 180 degree phase shifter.

A computer-implemented system for generating an alternating PSM designis also provided. The system includes an input interface for receiving alayout and an output interface for outputting the alternating PSMdesign. The system further includes the means for converting the layoutto the alternating PSM design. Notably, the means for convertingincludes software code for creating blockers in the 0 degree phaseshifters, thereby minimizing an intensity imbalance with theircorresponding 180 degree phase shifters.

The means for converting can further include software code forincreasing a dimension of the blocker to improve the intensityimbalance, software code for creating a uniform intensity imbalanceerror on the PSM design using a plurality of blockers, and/or softwarecode for performing optical proximity correction (OPC) on the PSMdesign. Note that OPC can be performed after or possibly even beforeincorporating the blockers in the PSM design.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a top view of an alternating PSM including closelyspaced opaque features formed on a transparent substrate.

FIG. 1B illustrates a cross sectional view of the alternating PSM ofFIG. 1A.

FIG. 1C illustrates a graph that plots intensity (0 to 1.0) versusposition on the alternating PSM of FIGS. 1A and 1B.

FIG. 1D illustrates a feature having a corresponding 0 degreephase-shifting region placed relative to one side of the feature and acorresponding 180 degree phase-shifting region placed relative to theother side of the feature.

FIG. 1E illustrates a cross sectional view of an alternating PSM inwhich an additional etching step under-cuts certain features.

FIG. 2 illustrates a simplified alternating PSM layout including a 0degree phase-shifting region and a corresponding 180 degreephase-shifting region usable for printing a feature (also shown forreference). To ensure that the feature prints on a wafer consistent witha desired centerline, a sub-resolution feature (called a “blocker”herein) can be positioned in the 0 degree phase-shifting region.

FIG. 3 illustrates a graph that plots relative intensity versusx-position for one pair of phase shifters. As shown in this graph, therelative intensity of the 0 degree phase shifter compared to the 180degree phase shifter decreases as the length of a blocker increases.

FIG. 4A illustrates a graph that depicts the rapid decrease of the spacecritical dimension (CD) bias as the length of a blocker increases.

FIG. 4B illustrates a graph that plots a printed line CD curve, a 0degree phase shifter CD curve, and a 180 degree phase shifter CD curvethrough a range of defocus settings.

FIG. 4C illustrates a graph that plots a space CD bias curve through thesame defocus settings of FIG. 4B. As shown by FIGS. 4A-4C, when usingblockers, through focus and printed line CD variation are withinacceptable limits, thereby providing satisfactory process latitude.

FIG. 4D illustrates a graph showing that the normalized image log slope(NILS) on either side of the intensity profile does not exhibit anysubstantial deterioration as intensity balance is achieved.

FIGS. 5A-5C illustrate cross sectional views of exemplary alternatingPSMs using blockers with other techniques to resolve intensityimbalance.

FIG. 6 illustrates an alternating PSM design flow implementable usingsoftware. In this flow, blockers can be used, as necessary, to minimizeintensity imbalance in the alternating PSM design.

FIG. 7 illustrates a system that can compensate for 3D effects on analternating PSM.

FIG. 8 illustrates an exemplary alternating PSM that can include aplurality of blockers within a 0 degree phase shifter.

FIG. 9 illustrates an exemplary alternating PSM that may significantlyimprove the intensity imbalance between corresponding phase shifters aswell as the straightness/uniformity of the resulting printed feature.

DETAILED DESCRIPTION OF THE FIGURES

To correct 3D effects while ensuring mechanical stability andmanufacturing efficiency, sub-resolution features can be placed withinthe 0 degree phase shifters of an alternating PSM, thereby substantiallyequalizing the intensity of the 0 degree and 180 degree phase shifters.For example, FIG. 2 illustrates a simplified alternating PSM layout 200.Layout 200 could be in, for example, a GDS-II format or any other formatproviding feature information regarding one or more layers of anintegrated circuit. In this simplified layout, a 0 degree phase shifter203 and an associated 180 degree phase shifter 204 can, when exposed,create a feature 201 (e.g. a transistor gate). To ensure that feature201 prints on a wafer consistent with a centerline 202, a sub-resolutionfeature (hereinafter “blocker”) 205 can be positioned in 0 degree phaseshifter 203.

Of importance, sub-resolution features, such as blocker 205, will notprint, i.e. resolve, on the wafer using standard PSM illuminationconditions. PSM illumination conditions can include, for example, anumerical aperture (NA) of 0.7-0.9 and a partial coherence (σ) of0.2-0.4. Advantageously, such PSM illumination conditions (particularlythe lower a values) allow a relatively large margin for thesub-resolution width, thereby allowing substantial flexibility inachieving the desired degree of intensity imbalance correction.

In one embodiment, the blocker width can be fixed and a minimum blockerlength 206 can be increased by predetermined increments (shown by arrows207) until maximum intensity balance is achieved for a specific pair ofphase shifters. For example, the width, minimum length, andpredetermined increment could be set to 20 nm, wherein the originalposition of the blocker could be centered in the 0 degree phase shifter.In one embodiment, when the length of the blocker reaches 140 nm, theincrement size can be increased to 40 nm. Logically, the length of the 0degree phase shifter determines the maximum length of the blocker. Inone embodiment, blocker 205 can be “grown” in 2× increments in twodirections (e.g. one increment “up” and another increment “down”). Inanother embodiment, blocker 205 can be grown in single increments in asingle direction determined by simulation results.

FIG. 3 illustrates a graph 300 that plots relative intensity (y-axis)versus position (x-axis) for one pair of phase shifters. Specifically,curves 301 represent the relative intensities associated with a 0 degreephase shifter (e.g. phase shifter 203) and curves 302 represent therelative intensities associated with a 180 degree phase shifter (e.g.phase shifter 204). The relative intensity can be defined as a “slice”of the intensity that is computed parallel to the plane of thealternating PSM. An arrow 303 indicates that the relative intensity ofthe 0 degree phase shifter decreases as the length of a blockerincreases (different blocker sizes are represented by different stipplepatterns in graph 300). A line 304 indicates the point at which theblock length (in this case, 250 nm) results in substantially equalrelative intensity for both the 0 degree and 180 degree phase shifters.Of importance, while the maximum intensity decreases the minimumintensity also simultaneously decreases, thereby advantageouslyminimizing any loss in contrast.

One of the ways to characterize the intensity imbalance is to calculatethe critical dimension (CD) difference or bias between the neighboring 0degree and 180 degree phase shifters. Note that the term “space CD” inthe context of phase shifters refers to the width of the phase shifter(e.g. width 208 of phase shifter 204). The difference between the CDs ofa complementary pair of phase shifters, called the space CD bias herein,decreases as the intensity imbalance is being corrected with a blocker.

For example, FIG. 4A illustrates a graph 401 that depicts the rapiddecrease of the space CD bias as the length of a blocker increases. Inthe described exemplary blocker, which is 20 nm wide, the space CD biasconverges to approximately 2 nm when the length of the blocker isapproximately 200 nm. In one embodiment, the width of the blocker can beslightly increased to, for example, a width less than 50 nm (note thatthe upper limit of this width is functionally dependent on imagingconditions (e.g. NA, sigma, wavelength), but would still not result in aprintable feature), thereby improving intensity balance and allowing thespace-CD bias to converge to zero.

In general, intensity imbalance can persist through different defocussettings. Specifically, if the intensity imbalance is significant atnominal focus, significant deterioration would typically occur in theprocess latitude through focus. Advantageously, after correction withblockers, through focus improvement can be achieved.

For example, FIG. 4B illustrates a graph 402 that plots a printed lineCD curve 403, a 0 degree phase shifter CD curve 404, and a 180 degreephase shifter CD curve 405 through defocus settings of −0.3 to 0.3.(Note that shifter CDs can vary due to 3D topography effects.) Notably,between defocus settings of −0.1 and 0.1, the printed line CD variesonly by 2-3 nm and achieves its lowest CD values.

FIG. 4C illustrates a graph 410 that plots a space CD bias curve 411through these same defocus settings (i.e. of −0.3 to 0.3). Note thatbetween defocus settings of −0.1 and 0.1, the intensity is balanced (asevidenced by the relative space CD bias being +/−10 nm). Thus, whenusing blockers, through focus and printed line CD variation are withinacceptable limits, thereby providing satisfactory process latitude.

To verify that the image quality is maintained, a normalized image logslope (NILS) can be measured. FIG. 4D shows a graph 420 demonstratingthat the NILS on either side of the intensity profile (shown by 0 degreephase shifter NILS curve 421 and 180 degree phase shifter NILS curve422) do not exhibit any substantial deterioration as intensity balanceis achieved. In fact, there is an increase in the NILS associated withboth curves 421 and 422, which indicates that blockers could actuallyenhance the image quality.

At the 65 nm technology node and below, intensity imbalance could poseadditional challenges and therefore may require supplemental correctiontechniques. The use of blockers within the 0 degree phase shifters canbe easily incorporated into such multi-faceted approaches whileminimizing PSM manufacturing complexity.

FIGS. 5A-5C illustrate cross-sectional views of exemplary alternatingPSMs using blockers with other techniques to resolve intensityimbalance. For example, FIG. 5A illustrates an alternating PSM mask 501that includes blockers 502 as well as undercuts 503 in the 180 degreephase shifters 504. FIG. 5B illustrates an alternating PSM mask 510 thatincludes blockers 512 as well as biasing 513 for the 180 degree phaseshifters 514. FIG. 5C illustrates an alternating PSM mask 520 thatincludes blockers 522 as well as undercuts 523 in and biasing 521 forthe 180 degree phase shifters 524.

Note that minimal undercutting in 180 degree phase shifting regions maybe desirably created in some standard PSM manufacturing processes.Specifically, in a typical etching process for the 180 degree phaseshifters of an alternating PSM, two etches are performed. The first etchis an anisotropic etch that provides a substantially vertical profile.However, this anisotropic etch can result in some jagged edges in theformed trench. Therefore, the second etch is an isotropic etch thatplanarizes those jagged edges. Notably, this isotropic etch results inundercut profiles for the 180 degree phase shifters.

Therefore, in one embodiment, the amount of undercutting can beadvantageously limited to that necessary to form a “clean” alternatingPSM having minimal jagged edges in its trenches while at the same timeproviding some minimal intensity imbalance correction. That is, theblockers can correct for the majority of the intensity imbalance createdby the 0 degree and 180 degree phase shifters and generate a uniformintensity imbalance error across the PSM. In this case, the undercuttingresulting from the second etch can supplement the intensity imbalancecorrection provided by the blockers. Specifically, the second etch cannow be performed globally on the alternating PSM to exactly correct forthe remaining (now uniform) intensity imbalance. Therefore, blockers caneliminate the overcompensation for intensity imbalance present in priorart mask-based solutions. Moreover, because undercutting is minimal,mechanical stability of the patterned chrome on the PSM is improved.

Advantageously, blockers can be easily incorporated into a standard PSMmanufacturing process. Specifically, blockers can be formed in theopaque (e.g. chrome) layer at the same time as other opaque features.Therefore, blockers ensure that manufacturing resources are efficientlyused.

Notably, the size, number of, or even the need for blockers can beeasily determined for each printable feature. For example, FIG. 6illustrates an alternating PSM design flow 600 implementable usingsoftware. In flow 600, a desired layout can be received in step 601. Instep 602, the desired layout can be converted into an alternating PSMdesign. At this point, blockers can be incorporated into the alternatingPSM design, as necessary, in step 603. Note that blocker incorporationcan be rule-based and/or determined by simulation. In step 604, opticalproximity correction (OPC) can be performed on the alternating PSMdesign including blockers, thereby facilitating a “fine tuning” of thealternating PSM design and even of the blockers themselves. In oneembodiment, steps 603 and 604 can be switched, i.e. blockers can beincorporated into the alternating PSM design first corrected for OPC. Inanother embodiment, steps 603 and 604 can be combined into a single stepin which model-based simulation can determine both OPC corrections andblocker inclusion for the alternating PSM design. Note that othercorrections can be performed as part of OPC, e.g. microloadingcorrections etc., can be performed after step 604. Microloadingcorrections are described in U.S. patent application Ser. No.09/945,012, which was filed on Aug. 31, 2001, published as US PatentPublication 2003/0046653 A1, by the assignee of the present application,Numerical Technologies, Inc. In one embodiment, corrections to thealternating PSM design can be performed in an order dependent on impacton printability, wherein the correction having the most impact onprintability could be performed first.

In some embodiments the iN-Phase® software and/or the Proteus™ softwarefrom the parent corporation of Numerical Technologies, Inc., Synopsys,Inc., can be suitably modified to support the steps of process 600. Forexample, as part of step 602, the iN-Phase® software can read in aGDS-II layout and convert such a layout to a PSM design. Similarly, theProteus™ software could be used at step 604. Either tool, whenappropriately modified, or a separate tool could perform step 603.

FIG. 7 illustrates a system 700 that can compensate for 3D effects on analternating PSM. In one embodiment, system 700 can include an inputinterface 701 for receiving a layout and an output interface 702 forproviding a modified layout that includes blockers. System 700 can alsoinclude a memory 704 that stores a plurality of computer-implementedprograms 703(1)-703(N) for implementing the steps described in referenceto FIG. 6 (e.g. layout conversion, blocker incorporation, OPC, etc.). Ina typical embodiment, system 700 can further include a processor 705 forexecuting computer-implemented programs 703.

Note that computer-implemented programs 703 can be run on any number ofcomputer platforms including: a PC using a UNIX®-like operating systemwith suitable memory and processor speeds, either stand alone orconnected to a network, and a SUN™ workstation computer among others. Inone embodiment, computer-implemented programs 703 include the EM-Suite™simulation program from Panoramic Technology that is based on thefinite-difference-time-domain algorithm TEMPEST. In another embodiment,computer-implemented programs 703 can include any software capable ofsolving rigorous Maxwell equations, thereby able to accurately take intoaccount the depth of the substrate and diffraction effects within thesubstrate (i.e. 3D effects).

Although illustrative embodiments of the invention have been describedin detail herein with reference to the accompanying figures, it is to beunderstood that the invention is not limited to those preciseembodiments. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed. As such, many modificationsand variations will be apparent. For example, FIG. 8 illustrates analternating PSM 800 that includes a 0 degree phase shifter 801 that issignificantly larger than its corresponding 180 degree phase shifter802. In this configuration, to more completely correct for intensityimbalance, a plurality of blockers (e.g. blockers 803 and 804) can beplaced within 0 degree phase shifter 801.

Note that placing identical sub-resolution features within both the 0degree and 180 degree shifters is taught by U.S. Pat. No. 6,541,165,which issued on Apr. 1, 2003 to the assignee of the present application,Numerical Technologies, Inc., to improve the straightness and uniformityof the resulting printed lines. In contrast, the blockers describedherein address the problem of intensity imbalance.

Therefore, in another embodiment shown in FIG. 9, an exemplaryalternating PSM 900 can include sub-resolution features placed in boththe 0 degree and 180 degree phase shifters to address intensityimbalance as well as straightness/uniformity. Specifically, alternatingPSM 900 can include a blocker 903 in a 0 degree shifter 901 that issized larger than a sub-resolution feature 904 in a 180 degree phaseshifter 902. In this manner, the intensity imbalance between the twoshifters as well as the straightness/uniformity of the resulting printedlines can be significantly improved.

In yet another embodiment, because of the inclusion of blockers, the 0degree phase shifters may be sized larger than the 180 degree phaseshifters during PSM conversion (e.g. step 602 of FIG. 6). In thismanner, the size of the blocker may be increased enough to improveintensity imbalance while still remaining a sub-resolution feature.

Note that although 0 degree phase shifters and 180 degree phase shiftersare discussed herein, blockers can be used with other phase shifterpairs having a phase shift differential of approximately 180 degrees(e.g. 10 degree phase shifters and 190 degree phase shifters).Therefore, the designation herein of a 0 degree or a 180 degree phaseshifter is merely indicative of the phase shift between a pair ofshifters and is not limiting.

Note that the methods described herein can be applied to a variety oflithographic process technologies, including ultraviolet, deepultraviolet (DUV), extreme ultraviolet (EUV), and x-ray. Accordingly, itis intended that the scope of the invention be defined by the followingclaims and their equivalents.

1. A method of designing an alternating phase shifting mask (PSM), themethod comprising: converting a layout to an alternating PSM designincluding 0 degree phase shifters and 180 degree phase shifters; andincorporating blockers in the alternating PSM design, wherein a blockeris formed in a 0 degree phase shifter to minimize an intensity imbalancewith its corresponding 180 degree phase shifter.
 2. The method of claim1, wherein incorporating blockers includes growing a length of theblocker.
 3. The method of claim 1, wherein incorporating blockersincludes forming a plurality of blockers in the 0 degree phase shifter.4. The method of claim 1, wherein if a 180 degree phase shifter includesa sub-resolution feature, then sizing the blocker in the 0 degree phaseshifter to be larger than the sub-resolution feature.
 5. The method ofclaim 1, wherein incorporating blockers creates a substantially uniformintensity imbalance error on the alternating PSM.
 6. The method of claim1, further including performing optical proximity correction (OPC) onthe alternating PSM design.
 7. The method of claim 6, wherein performingOPC is done after incorporating blockers in the alternating PSM design.8. The method of claim 6, wherein performing OPC is done beforeincorporating blockers in the alternating PSM design.
 9. A method ofgenerating an alternating phase shifting mask (PSM) design including 0degree phase shifters and 180 degree phase shifters, the methodcomprising: minimizing an intensity imbalance between a 0 degree phaseshifter and a 180 degree phase shifter corresponding to the 0 degreephase shifter by incorporating a first sub-resolution feature in the 0degree phase shifter.
 10. The method of claim 9, wherein incorporatingthe first sub-resolution feature includes growing a single dimension ofthe first sub-resolution feature.
 11. The method of claim 9, whereinincorporating the first sub-resolution feature includes forming aplurality of sub-resolution features in the 0 degree phase shifter. 12.The method of claim 9, wherein if the 180 degree phase shifter includesa second sub-resolution feature, then sizing the first sub-resolutionfeature to be larger than the second sub-resolution feature.
 13. Themethod of claim 9, wherein incorporating the first sub-resolutionfeature for each 0 degree phase shifter and 180 degree phase shifter ofthe alternating PSM creates a substantially uniform intensity imbalanceerror on the alternating PSM.
 14. The method of claim 9, furtherincluding performing optical proximity correction (OPC) on thealternating PSM design.
 15. The method of claim 14, wherein performingOPC is done after incorporating minimizing intensity imbalance.
 16. Themethod of claim 14, wherein performing OPC is done before incorporatingminimizing intensity imbalance.
 17. An alternating phase shifting mask(PSM) comprising: a 180 degree phase shifter; a 0 degree phase shiftercorresponding to the 180 degree phase shifter; and at least onesub-resolution feature formed in the 0 degree phase shifter to minimizean intensity imbalance with the 180 degree phase shifter.
 18. Thealternating PSM of claim 17, further including an undercut in the 180degree phase shifter.
 19. The alternating PSM of claim 17, furtherincluding a bias in the 180 degree phase shifter.
 20. The alternatingPSM of claim 17, further including a sub-resolution feature formed inthe 180 degree phase shifter, wherein the sub-resolution feature formedin the 0 degree phase shifter is larger than the sub-resolution featureformed in the 180 degree phase shifter.
 21. A computer-implementedsystem for generating an alternating phase shifting mask (PSM) design,the alternating PSM design including 0 degree phase shifters and 180degree phase shifters, the system comprising: an input interface forreceiving a layout; means for converting the layout to the alternatingPSM design, wherein the alternating PSM design includes a blocker formedin a 0 degree phase shifter to minimize an intensity imbalance with acorresponding 180 degree phase shifter; and an output interface foroutputting the alternating PSM design.
 22. The computer-implementedsystem of claim 21, wherein the means for converting includes softwarecode for increasing a dimension of the blocker to improve the intensityimbalance.
 23. The computer-implemented system of claim 21, wherein themeans for converting includes software code for creating a uniformintensity imbalance error on the alternating PSM using a plurality ofblockers.
 24. The computer-implemented system of claim 21, wherein themeans for converting includes software code for performing opticalproximity correction (OPC) on the alternating PSM design.
 25. A methodof manufacturing an alternating phase shifting mask (PSM), the methodcomprising: using a software-implemented technique to create a uniformintensity imbalance error on the alternating PSM; and using amask-implemented technique to correct for the uniform intensityimbalance error on the alternating PSM.